The most common target for mutations in tumours is the p53 gene. The fact that around half of all human tumours carry mutations in this gene is solid testimony as to its critical role as tumour suppressor p53 halts the cell cycle and/or triggers apoptosis in response to various stress stimuli, including DNA damage, hypoxia, and oncogene activation (Ko and Prives, 1996; Sherr, 1998). Upon activation, p53 initiates the p53-dependent biological responses through transcriptional transactivation of specific target genes carrying p53 DNA binding motifs. In addition, the multifaceted p53 protein may promote apoptosis through repression of certain genes lacking p53 binding sites, and transcription-independent mechanisms as well (Bennett et al., 1998; Gottlieb and Oren, 1998; Ko and Prives, 1996). Analyses of a large number of mutant p53 genes in human tumours have revealed a strong selection for mutations that inactivate the specific DNA binding function of p53; most mutations in tumours are point mutations clustered in the core domain of p53 (residues 94-292) that harbours the specific DNA binding activity (Beroud and Soussi, 1998).
Both p53-induced cell cycle arrest and apoptosis could be involved in p53-mediated tumour suppression. While p53-induced cell cycle arrest could conceivably be reversed in different ways, p53-induced cell death would have advantage of being irreversible. There is indeed evidence from animal in vivo models (Symonds et al., 1994) and human tumours (Bardeesy et al., 1995) indicating that p53-dependent apoptosis plays a major role in the elimination of emerging tumours, particularly in response to oncogenic signalling. Moreover, the ability of p53 to induce apoptosis often determines the efficacy of cancer therapy (Lowe et al., 1994). Taking into account the fact that more than 50% of human tumours carry p53 mutations, it appears highly desirable to restore the function of wild type p53-mediated growth suppression to tumours. The advantage of this approach is that it will allow selective elimination of tumour cells carrying mutant p53. Tumour cells are particularly sensitive to p53 reactivation, supposedly for two main reasons. First, tumour cells are sensitized to apoptosis due to oncogene activation (reviewed in (Evan and Littlewood, 1998)). Second, mutant p53 proteins tend to accumulate at high levels in tumour cells. Therefore, restoration of the wild type function to the abundant and presumably “activated” mutant p53 should trigger a massive apoptotic response in already sensitized tumour cells, whereas normal cells that express low or undetectable levels of p53 should not be affected. The feasibility of p53 reactivation as an anticancer strategy is supported by the fact that a wide range of mutant p53 proteins are susceptible to reactivation. A therapeutic strategy based on rescuing p53-induced apoptosis should therefore be both powerful and widely applicable.
Taken together, these findings strongly suggest that pharmacological restoration of p53 function would result in elimination of tumour cells. Consequently, there is a need within this field to identify substances and methods that will enable such restoration of p53 function.
For the above defined purpose, it has been shown that p53 is a specific DNA binding protein, which acts as a transcriptional activator of genes that control cell growth and death. Thus, the ability of the p53 protein to induce apoptosis is dependent on its specific DNA binding function. Mutant p53 proteins carrying amino acid substitutions in the core domain of p53, which abolish the specific DNA binding, are unable to induce apoptosis in cells. Therefore, in order to obtain such substances and methods as defined above, reactivation of p53 specific DNA binding is essential in order to trigger p53-dependent apoptosis in tumours during pathological conditions.